Winter seasonal affective disorder (SAD) is a subtype of recurrent depression involving major depressive episodes during the fall and/or winter months that remit each spring. The central public health challenge in the management of SAD is prevention of winter depression recurrences. This application focuses on two SAD treatments that each work for some patients: light therapy (LT) and a SAD-tailored group cognitive-behavioral therapy (CBT-SAD). LT is the acute SAD treatment with the most substantial evidence to support its efficacy. Correction of circadian phase is LT's established target and mechanism. In our recently completed R01-level efficacy trial, post-treatment outcomes for CBT-SAD and LT were very similar, but CBT-SAD was associated with fewer depression recurrences over 2-year followup than LT (27.3% in CBT- SAD vs. 45.6% in LT). CBT-SAD engaged and altered a specific mechanism of action, seasonal beliefs, which improved at twice the rate during CBT-SAD compared to LT, and this improvement was associated with lower risk for recurrence following CBT-SAD. This confirmatory efficacy R01 will apply the experimental therapeutics approach to determine how each treatment works when it is effective and to identify the best candidates for each. We will ascertain whether theoretically-derived candidate biomarkers of each treatment's target and effect are prescriptive of better outcomes in that treatment vs. the other. Biomarkers of LT's target and effect include circadian phase angle difference (PAD) and the post-illumination pupil response (PIPR). Biomarkers of CBT-SAD's target and effect include pupil dilation and sustained gamma band EEG responses to seasonal words, which are hypothesized to reflect less engagement with seasonal stimuli following CBT-SAD and corroborate with the established target of seasonal beliefs. In addition to determining change mechanisms, we will test the efficacy of a ?switch? decision rule upon recurrence to inform clinical decision-making in practice. We will randomize 160 adults with SAD to 6-weeks of CBT-SAD or LT in Winter 1; follow subjects in Winter 2; and, if a depression recurrence occurs, cross them over into the alternate treatment (i.e., switch from LT?CBT-SAD or CBT-SAD?LT). All subjects will be followed in Winter 3. Biomarker assessments will occur at pre- and post-treatment in Winter 1, at Winter 2 followup (and again at post-treatment for those crossed-over), and at Winter 3 followup. Consistent with NIMH's priorities for demonstrating target engagement at the level of RDoC-relevant biomarkers, this work aims to confirm the targets and mechanisms of LT and CBT-SAD to maximize the impact of future dissemination efforts.